Thermosetting urea-formaldehyde composition



Patented Jan. 7, 1947 THERMOSETTING UREA-FORMALDEHYDE COIVIPOSITION David E. Cordier, Toledo, Ohio, assignor, by mes ne assignments,

to Libbey-Owens-Ford Glass Company, a corporation of Ohio No Drawing. Application March 19, 1943,

Serial No. 479,743

3 Claims 1 The invention relates to a thermosetting composition comprising a urea-formaldehyde reaction product and a novel latent curing catalyst.

When a thermosetting composition is shaped in a mold under pressure at an elevated temperature, it first softens and then hardens at the molding temperature, whereas a thermoplastic com-- position that is hot-molded remains soft until the molded piece is cooled. Thus an article can be molded from a thermosetting composition by simplyplacing the composition in a hot mold, closing the mold, and then removing the finished article after a relatively short time while the mold is still hot. The molding of an article from :1 thermoplastic composition presents a more complex problem, because an article molded from such a composition must be cooled beforethe mold is opened to preventblistering and other deformation which would occur if the 'mold were opened.

while the thermoplastic material was still hot.

The length of time for which a molding composition must be left in the mold is one of the be left in the mold twice as long as another composition requires about twice as much molding equipment for .the same volume of production. and hot-molding equipment is expensive.

A urea-formaldehyde reaction product is thermosetting inthe presence of an acid substance in that'itis cured or transformed by heat from a fusible composition into an infusible resin. In order to cause such transformation to take place, an acid substance must .be present to act as a catalyst. In the molding of an article from a urea-formaldehyde composition, it is necessary to leave the composition in the hot mold for .a short time after the mold has been closed in order to complete the transformation to the infusible resin so as to produce an article of optimum quality. The length of time for which it is necessary to leave a urea-formaldehyde composition in the mold varies with the degree of acidity produced bythe acid substance used as a catalyst: the more acid the composition, the

.shorter the time required to complete the transformation to an infusible resin in the hot mold. There is, of course, a-demand for urea-formalde hyde molding compositions that are completely converted into an infusible resin so that the finished article can be removed from the mold a relatively short time after the mold has been closed.

- factors determining the cost of articles molded I from the composition. A composition that must comes infusible at ordinary temperatures, Such a composition could not be sold by a manufacturer because it would be infusible andworthless by the time it reached the hands of a molder.

Even slight acidity which causes the composition to become infusible very gradually would make the composition commercially unacceptable. because the plasticity and other molding properties of such a composition would be wholly dependent upon the atmospheric temperature prevailiirg and the number of hours .elapsed between the production of the finished composition by the manufacturer and the molding of the composition by the user. A user who molds articles from a urea-formaldehyde composition must select a composition of the proper plasticity and test the composition by molding it under various conditions to determine the exact conditions required to give the desired results. In order to maintain the quality of the product, he must then continue to use a composition of exactly the same plasticity and to mold it under exactly the same conditions. If the composition used by the molder is unstable and has molding properties which vary with the length of time elapsed since the composition was produced, it is impossible for the molder to mold successive articles from the composition with satisfactory results.

A latent curing catalyst in a urea-formaldehyde composition is an ingredient that causes the transformation of the composition to an infusible resin in a hot mold, but does not materially impair the stability of the composition in storage at ordinary temperatures prior to hot molding. True latent curing catalysts are very rare. latent curing catalyst may be alkaline, neutral or so slightly acid that it does not appreciably acidify a urea-formaldehyde composition when added thereto. It is believed that such a substance breaks up orundergoes molecular rearrangement to form an acid, but does not do so until the molding temperature is reached. substance may act as a curing catalyst the acid so formed must be strong enough to cause the transformation of the urea-formaldehyde reaction product into an infusible resin.

A urea-formaldehyde molding composition contains an appreciable amount of moisture and frequently is kept in storage for weeks at a time before being used. Many substances that might be expected to decompose to form acids under molding conditions suffer the same decomposition within a few hours after being intimately mixed with a urea-formaldehydecomposition, and therefore are not latent curing catalysts. Moreover, the behavior of a substance when present as a minor ingredient in a molding composition and subjected to molding pressure at the molding temperature of 270 to 330 F. cannot be predicted from its behavior when subjected by itself In order that the that liberate organic acids are preferable .to substances that liberate strong inorganic acids, because of the danger of mold corrosion by strong inorganic acids. Certain organic peroxides, such as benzoyl peroxide, have been used heretofore as latent curing catalysts, but there are many organic pigments useful in molding compositions that are deleteriousiy affected when a peroxide is present in the composition.

The principal object of the invention is to provide a thermosetting urea-formaldehyde composition containing a novel latent curing catalyst. More specific objects and advantages are apparent from the description, which discloses and illustrates the invention, and is not intended to impose limitations upon the claims.

A thermosetting composition embodying the invention comprises a urea-formaldehyde reaction product and dibenzyl oxalate (ocnroiiah c) as a latent curing catalyst. Dibenzyl oxalate is slightly acid, but when a perfectly neutral composition is desired, even the slight acidity of the dibenzyl oxalate may be neutralized by incorporation of a small amount of an alkaline substance in the composition. The dibenzyl oxalate is not decomposed by neutralization, and in this respect it differs from many acid substances which are capable of thermal decomposition into products of increased acidity but which decompose at room temperature when an attempt is made to neutralize them. a

In the preparation of a reaction product of urea and formaldehyde for use in a composition embodying the invention, the urea may be reacted either with formaldehyde or with a polymer thereof, such as paraformaldehyde. Although under some conditions it is permissible to react dry urea with dry paraformaldehyde, the reaction that is approximatelyneutral at the start of the reaction. Since commercial aqueous formaldehyde solution is strongly acid, a base is preferably added to bring the initial pH of the reaction solution to the desired value. Any desired base such as sodium or potassium hydroxide or any weaker base, or an organic base such as triethanolamine may be employed. The preferred proportion of the reactants is three mols of formaldehyde for two mols of urea. Approximately two mols of formaldehyde are all that will react with each mol of urea, but an excess of formaldehyde above such maximum or a smaller proportion ranging down to about one mol of formaldehyde for each mol of urea may be used for the reaction if desired. Because of the complexity of the molecules of the reaction products that are produced, the proportion of formaldehyde actually reacting with the urea may vary freely between the limits stated. The reaction proceed at ordinary temperatures, but heat may be used to shorten the time of reaction if desired. A reaction product may be prepared by carrying the reaction of the urea and formaldehyde only to its earliest stage, for example the stage at which the urea and formaldehyde have just been brought into solution together, or the reaction may be carried to any further stage at which the reaction product is still fusible.

The preferred method of preparing a molding composition consists in preparing an aqueous solution of a urea-formaldehyde reaction product, impregnating cellulosic material with the solution, and then drying. Although alpha cellulose is the purest and lightest-colored cellulosic material that may be employed, any other cellulosic I material, such as wood flour, wood pulp, newsprint, printed newspaper, sawdust, shavings, wainut shell flour, or ground corn cobs may be used.-

The impregnated and dried cellulosic material is preferably ground to a fine powder in order to produce a homogeneous composition, and the latent curing catalyst is preferably incorporated during the grinding stage. The customary modifiers such as hot-plate lubricants, opaciflers, pigments and other coloring matter may also be incorporated during the grinding. The fine powder so obtained may be formed into coarse granules, or into solid blanks orpreforms of the proper sizes for use in various molds. Molded articles may be produced in the usual manner by compressing the composition in a closed mold under a pressure of one to four tons per square inch of projected area and at a temperature of 270-330 F. The proportion of cellulosic material in a dry composition embodying the invention is preferably from about 30 to about 40 per cent, but may range from 0 per cent to a much as 60 per cent in the case of a dense cellulosic material, such as walnut shell flour. The proportion of the latent curing catalyst employed is simply that proportion which causes the hardening to take place at the desired speed, but the usual proportion is from about .4-, to about /2 of one per cent of the weight of the molding composition.

Example After alpha cellulose fiber (80 parts by weight) has been impregnated with an aqueous solution containing 120 parts of a urea-formaldehyde reaction product, the impregnated material is dried by any of the usual drying methods.

Heat may be used as is customary to expedite the drying,

and drying by means of, a stream of air is contion is preferably carried out in anaqueous soluvenient.- The dried material is ground in a ball 'mill together with 0.4 percent of its weight of dibenzyl oxalate, and any other desired modifiers. The'resulting powder is usable as a molding composition for many applications but can be granulated or preformed.

Various compositions embodying the invention I half of one per, cent of the weight of the composition, as a latent curingcatalyst.

DAVID) E. CORDIER. 

